Vertical gate all around (VGAA) devices and methods of manufacturing the same

ABSTRACT

Vertical gate all around (VGAA) devices and methods of manufacture thereof are described. A method for manufacturing a VGAA device includes: exposing a top surface and sidewalls of a first portion of a protrusion extending from a doped region, wherein a second portion of the protrusion is surrounded by a gate stack; and enlarging the first portion of the protrusion using an epitaxial growth process.

PRIORITY CLAIM AND CROSS-REFERENCE

This application is a continuation of U.S. patent application Ser. No. 15/872,361, entitled “Vertical Gate All Around (VGAA) Devices and Methods of Manufacturing the Same,” filed on Jan. 16, 2018, which is a divisional of U.S. patent application Ser. No. 14/660,542, entitled “Vertical Gate All Around (VGAA) Devices and Methods of Manufacturing the Same,” filed on Mar. 17, 2015 (now U.S. Pat. No. 9,899,489, issued Feb. 20, 2018), which is a continuation-in-part of U.S. patent application Ser. No. 14/621,628, entitled “Vertical Gate All Around (VGAA) Devices and Methods of Manufacturing the Same,” filed on, Feb. 13, 2015, which patent applications are incorporated herein by reference.

BACKGROUND

As the semiconductor industry has progressed into nanometer technology nodes in pursuit of higher device density, higher performance, and lower costs, challenges from both fabrication and design issues have resulted in the development of three-dimensional designs, such as a vertical gate all around (VGAA) transistor. A typical VGAA transistor enables enhanced control of the charge carriers along the lengthwise direction through a complete encirclement of the channel region of a semiconductor nanowire by a gate dielectric and a gate electrode. The VGAA transistor has a reduced short channel effect (e.g. compared to a planar transistor), because the channel region may be surrounded by the gate electrode so that an effect of the source/drain region on an electric field of the channel region may be reduced (e.g. relative to a planar transistor).

However, VGAA transistors may suffer from high contact resistance and high parasitic capacitances. As such, improvements are needed in the manufacturing processes in order to manufacture VGAA transistors with lower contact resistances and lower parasitic capacitances.

BRIEF DESCRIPTION OF THE DRAWINGS

Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion.

FIGS. 1A to 1H show a process flow illustrating various intermediary steps of manufacturing a semiconductor device having a first vertical gate all around (VGAA) device and a second VGAA device, in accordance with one or more embodiments.

FIGS. 2A to 2D show top-down and cross-sectional views of drain layers and enlarged drain regions extending from surfaces of the drain layers, in accordance with one or more embodiments.

FIGS. 3A and 3B show top-down views illustrating various shapes of protrusions, in accordance with one or more embodiments.

DETAILED DESCRIPTION

The following discl osure provides many different embodiments, or examples, for implementing different features of the provided subject matter. Specific examples of components and stacks are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed.

Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature's relationship to another element(s) or feature(s) as illustrated in the figures. The spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly.

FIGS. 1A to 1H show a process flow illustrating various intermediary steps of manufacturing a semiconductor device 100 having a first vertical gate all around (VGAA) device 102 and a second VGAA device 202, in accordance with one or more embodiments. As an example, the first VGAA device 102 may be an NMOS VGAA device, while the second VGAA device 202 may be a PMOS VGAA device.

The semiconductor device 100 may comprise a semiconductor substrate 104 over which the first VGAA device 102 and the second VGAA device 202 are formed. The semiconductor substrate 104 may be a semiconductor wafer and may comprise silicon (Si); germanium (Ge); a compound semiconductor including silicon carbide, gallium arsenic, gallium phosphide, indium phosphide, indium arsenide, and/or indium antimonide; an alloy semiconductor including SiGe, GaAsP, AlInAs, AlGaAs, GaInAs, GaInP, and/or GaInAsP; or combinations thereof. The semiconductor substrate 104 may be a bulk semiconductor substrate, a semiconductor-on-insulator (SOI) substrate, a multi-layered or gradient semiconductor substrate, or the like.

The semiconductor device 100 may include a first doped region 106 and a second doped region 206 laterally adjacent to and separated from the first doped region 106. The first doped region 106 may be a part or a portion of the first VGAA device 102, while the second doped region 206 may be a part or a portion of the second VGAA device 202. The first doped region 106 may have a first conductivity, while the second doped region 206 may have a second conductivity different from the first conductivity. For example, as described above, the first VGAA device 102 and the second VGAA device 202 may be an NMOS VGAA device and a PMOS VGAA device, respectively. In such an embodiment, the first doped region 106 may comprise a semiconductor material (such as silicon, germanium, silicon germanium, combinations of these, or the like) that contains N-type dopants (such as phosphorous or arsenic), while the second doped region 206 may comprise a semiconductor material (such as silicon, germanium, silicon germanium, combinations of these, or the like) that contains P-type dopants (such as boron or gallium).

The first doped region 106 and the second doped region 206 may be separated from each other by an isolation feature 108 (e.g. shallow-trench isolation feature). The isolation feature 108 may comprise an insulating material such as a dielectric material (e.g. silicon oxide or the like) and may serve to electrically isolate the first VGAA device 102 and the second VGAA device 202 from each other. The isolation feature 108 may be formed between the first doped region 106 and the second doped region 206 by etching a recess in the first doped region 106 and/or the second doped region 206 and filling the recess with the insulating material using, for example, a spin-on-dielectric (SOD) process, or any other suitable process.

The semiconductor device 100 may include first protrusions 110 (e.g. disposed over and extending from the first doped region 106) and second protrusions 210 (e.g. disposed over and extending from the second doped region 206). The first protrusions 110 may be a part or a portion of the first VGAA device 102, while the second protrusions 210 may be a part or a portion of the second VGAA device 202. In the embodiment shown in FIG. 1A, only two first protrusions 110 and only two second protrusions 210 are shown as an example. However, in other embodiments, the number of first protrusions 110 and/or the number of second protrusions 210 may be less than two (e.g. one) or more than two (e.g. three, four, or more). The first protrusions 110 and/or the second protrusions 210 may be shaped as nanowires. In other words, the first protrusions 110 and/or the second protrusions 210 may have a substantially circular shape, e.g. when viewed in a top-down view. Alternatively, the first protrusions 110 and/or the second protrusions 210 may be shaped as bars or fins, e.g. when viewed in a top-down view. These embodiments are described below in greater detail in respect of FIGS. 3A and 3B.

Referring to the first VGAA device 102, each of the first protrusions 110 may be a multi-layer semiconductor substrate comprising a source layer 110 a, a channel layer 110 b, and a drain layer 110 c. In a particular embodiment, at least a portion of the source layers 110 a, the channel layers 110 b, and the drain layers 110 c of the first protrusions 110 form the source regions, channel regions, and drain regions of the first VGAA device 102, respectively.

The source layers 110 a, the channel layers 110 b, and the drain layers 110 c may comprise any suitable semiconductor material, such as silicon, germanium, silicon germanium, combinations of these, or the like. For example, in an embodiment, each of the source layers 110 a, the channel layers 110 b, and the drain layers 110 c comprise doped silicon. However, in another embodiment, the channel layers 110 b may comprise doped silicon, while the source layers 110 a and the drain layers 110 c comprise doped silicon germanium. In some embodiments, the source layers 110 a may be formed by patterning a portion (e.g. an upper portion) of the first doped region 106. Consequently, in such embodiments, the source layers 110 a may comprise a similar semiconductor material as the first doped region 106.

The semiconductor material of each of the source layers 110 a, the channel layers 110 b, and the drain layers 110 c also comprises dopants that cause the source layers 110 a, the channel layers 110 b, and the drain layers 110 c to have the same conductivity as the first doped region 106 (e.g. the first conductivity). For example, the first VGAA device 102 may be an NMOS VGAA device and, consequently, the source layers 110 a, the channel layers 110 b, and the drain layers 110 c of the first protrusions 110 may be doped with N-type dopants such as phosphorous or arsenic.

In an embodiment, the dopant concentration of the first doped region 106, the source layers 110 a, and the drain layers 110 c may be substantially equal to one another and may, at the same time, be greater than the dopant concentration of the channel layers 110 b. For example, the dopant concentration of the first doped region 106, the source layers 110 a, and the drain layers 110 c may be in a range from about 1×10¹⁹ cm ⁻³ to about 1×10²² cm ⁻³ or even greater, while the dopant concentration of the channel layers 110 b may be less than about 1×10¹⁸ cm⁻³. In such an embodiment, the drain layers 110 c may be highly doped drain regions of the first VGAA device 102.

In another embodiment, the dopant concentration of the first doped region 106 and the source layers 110 a may be substantially equal to each other and may, at the same time, be greater than the dopant concentration of the channel layers 110 b and the drain layers 110 c. For example, the dopant concentration of the first doped region 106 and the source layers 110 a may be in a range from about 1×10¹⁹ cm⁻³ to about 1×10²² cm⁻³ or even greater, while the dopant concentration of the channel layers 110 b and the drain layers 110 c may be less than about 1×10¹⁸ cm⁻³. In such an embodiment, the drain layers 110 c may lowly or moderately doped drain regions of the first VGAA device 102.

The first doped region 106 and the first protrusions 110 may be formed by epitaxially growing semiconductor material (e.g. using a first epitaxial growth process) over at least a portion of the semiconductor substrate 104 and subsequently patterning the grown semiconductor material (e.g. using an etching process) to form the first protrusions 110 extending from the first doped region 106. In some embodiments, the first epitaxial growth process may be molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), selective epitaxial growth (SEG), or combinations thereof. Other epitaxial growth processes may also be possible. As described above, the source layers 110 a may be formed by patterning a portion (e.g. an upper portion) of the first doped region 106. Consequently, the semiconductor material of the first doped region 106 and the source layers 110 a may be formed using the same epitaxial growth process. Furthermore, in some embodiments, the semiconductor material of each of the source layers 110 a, the channel layers 110 b, and the drain layers 110 c may be formed using the same epitaxial growth process. However, in another embodiment, different epitaxial growth processes may be used to form the semiconductor material of the source layers 110 a, the channel layers 110 b, and the drain layers 110 c of the first protrusions 110.

As described above, the first doped region 106 and the first protrusions 110 comprise doped semiconductor material. In an embodiment, dopants are introduced into the semiconductor material of the first doped region 106, the source layers 110 a, the channel layers 110 b, and the drain layers 110 c as the semiconductor material of each of these features is grown. As an example, during the epitaxial growth process of the semiconductor material of the first doped region 106, precursors that comprise the desired dopants are placed in situ into the reaction vessel along with the precursor reactants for the semiconductor material of the first doped region 106. As such, the dopants are introduced and incorporated into the semiconductor material of the first doped region 106 to provide the first doped region 106 the desired conductivity and dopant concentration while the semiconductor material of the first doped region 106 is grown. Although the example presented above is directed to the first doped region 106, a similar process may be used to introduce dopants into the semiconductor material of the channel layers 110 b and the drain layers 110 c as the semiconductor material of each of these layers is grown.

Alternatively, in another embodiment, dopants may be introduced into the semiconductor material of the first doped region 106, the source layers 110 a, the channel layers 110 b, and the drain layers 110 c after the semiconductor material of each layer is grown. As an example, the semiconductor material of the first doped region 106 may be grown without the dopants, and an introduction process such as an implantation process or diffusion process is utilized to introduce the dopants into the material of the first doped region 106 after this epitaxial growth process, but before growing the material of the channel layers 110 b. Once the dopants have been introduced into semiconductor material of the first doped region 106, an anneal process may be performed to activate the dopants. Thereafter, the epitaxial growth of the semiconductor material of the channel layers 110 b may be commenced. Although the example presented above is directed to the first doped region 106, a similar process may be used to introduce dopants into the semiconductor material of the channel layers 110 b, and the drain layers 110 c after the semiconductor material of each of these layers is grown.

Referring to the second VGAA device 202, each of the second protrusions 210 may be a multi-layer semiconductor substrate comprising a source layer 210 a, a channel layer 210 b, and a drain layer 210 c. In a particular embodiment, at least a portion of the source layers 210 a, the channel layers 210 b, and the drain layers 210 c of the second protrusions 210 form the source regions, channel regions, and drain regions of the second VGAA device 202, respectively.

The source layers 210 a, the channel layers 210 b, and the drain layers 210 c may comprise any suitable semiconductor material, such as silicon, germanium, silicon germanium, combinations of these, or the like. For example, in an embodiment, each of the source layers 210 a, the channel layers 210 b, and the drain layers 210 c comprise doped silicon. However, in another embodiment, the channel layers 210 b may comprise doped silicon, while the source layers 210 a and the drain layers 210 c comprise doped silicon germanium. In some embodiments, the source layers 210 a may be formed by patterning a portion (e.g. an upper portion) of the second doped region 206. Consequently, in such embodiments, the source layers 210 a may comprise a similar semiconductor material as the second doped region 206.

The semiconductor material of each of the source layers 210 a, the channel layers 210 b, and the drain layers 210 c also comprises dopants that cause the source layers 210 a, the channel layers 210 b, and the drain layers 210 c to have the same conductivity as the second doped region 206 (e.g. the second conductivity), which is different from the conductivity of the first doped region 106. For example, the second VGAA device 202 may be a PMOS VGAA device and, consequently, the source layers 210 a, the channel layers 210 b, and the drain layers 210 c of the second protrusions 210 may be doped with P-type dopants such as boron or gallium.

In an embodiment, the dopant concentration of the second doped region 206, the source layers 210 a, and the drain layers 210 c may be substantially equal to one another and may, at the same time, be greater than the dopant concentration of the channel layers 210 b. For example, the dopant concentration of the second doped region 206, the source layers 210 a, and the drain layers 210 c may be in a range from about 1×10¹⁹ cm ⁻³ to about 1×10²² cm⁻³ or even greater, while the dopant concentration of the channel layers 210 b may be less than about 1×10¹⁸ cm⁻³. In such an embodiment, the drain layers 210 c may be highly doped drain regions of the second VGAA device 202.

In another embodiment, the dopant concentration of the second doped region 206 and the source layers 210 a may be substantially equal to each other and may, at the same time, be greater than the dopant concentration of the channel layers 210 b and the drain layers 210 c. For example, the dopant concentration of the second doped region 206 and the source layers 210 a may be in a range from about 1×10¹⁹ cm⁻³ to about 1×10²² cm⁻³ or even greater, while the dopant concentration of the channel layers 210 b and the drain layers 210 c may be less than about 1×10¹⁸ cm⁻³. In such an embodiment, the drain layers 210 c may lowly or moderately doped drain regions of the second VGAA device 202.

The second doped region 206 and the second protrusions 210 may be formed by epitaxially growing semiconductor material (e.g. using a second epitaxial growth process) over at least a portion of the semiconductor substrate 104 and subsequently patterning the semiconductor material (e.g. using an etching process) to form the second protrusions 210 extending from the second doped region 206. In some embodiments, the second epitaxial growth process may be molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), selective epitaxial growth (SEG), or combinations thereof. Other epitaxial growth processes may also be possible. As described above, the source layers 210 a may be formed by patterning a portion (e.g. an upper portion) of the second doped region 206. Consequently, the semiconductor material of the second doped region 206 and the source layers 210 a may be formed using the same epitaxial growth process. Furthermore, in some embodiments, the material of each of the source layers 210 a, the channel layers 210 b, and the drain layers 210 c may be formed using the same epitaxial growth process. However, in another embodiment, different epitaxial growth processes may be used to form the material of the source layers 210 a, the channel layers 210 b, and the drain layers 210 c of the second protrusions 210.

As described above, the second doped region 206 and the second protrusions 210 comprise doped semiconductor material. In an embodiment, dopants are introduced into the semiconductor material of the second doped region 206 and the second protrusions 210 as the semiconductor material of each of these features is grown. The description given above in respect of introducing dopants into the semiconductor material of the first doped region 106, the source layers 110 a, the channel layers 110 b, and the drain layers 110 c as the semiconductor material of each of these layers is grown may analogously apply to introducing dopants into the semiconductor material of the second doped region 206, the source layers 210 a, the channel layers 210 b, and the drain layers 210 c as the semiconductor material of each of these layers is grown.

Alternatively, in another embodiment, dopants may be introduced into the semiconductor material of the second doped region 206 and the second protrusions 210 after the semiconductor material of each layer is grown. The description given above in respect of introducing dopants into the semiconductor material of the first doped region 106, the source layers 110 a, the channel layers 110 b, and the drain layers 110 c after the semiconductor material of each of these layers is grown may analogously apply to introducing dopants into the semiconductor material of the second doped region 206, the source layers 210 a, the channel layers 210 b, and the drain layers 210 c after the semiconductor material of each of these layers is grown.

As shown in FIG. 1A, the semiconductor device 100 may include first silicide regions 112 and second silicide regions 212. The first silicide regions 112 may be formed in portions of the first doped region 106 proximal the source layers 110 a of the first protrusions 110 and may be used for integrated device contacts to the source layers 110 a of the first VGAA device 102. As an example, the first silicide regions 112 may be disposed around the source layers 110 a of the first protrusions 110, e.g. when viewed in a top-down view. In like manner, the second silicide regions 212 may be formed in portions of the second doped region 206 proximal the source layers 210 a of the second protrusions 210 and may be used for integrated device contacts to the source layer 210 a of the second VGAA device 202. As an example, the second silicide regions 212 may be disposed around the source layers 210 a of the second protrusions 210, e.g. when viewed in a top-down view.

The first silicide regions 112 and the second silicide regions 212 may be formed using a silicide process or other suitable methods, e.g. after the patterning process that forms the first protrusions 110 and the second protrusions 210. The first silicide regions 112 and the second silicide regions 212 may comprise one or more metal species that can be used to form silicide compounds of the first silicide regions 112 and the second silicide regions 212. For example, the first silicide regions 112 and the second silicide regions 212 can comprise silicide compounds of titanium (e.g. TiSi₂), cobalt (e.g. CoSi₂), nickel (e.g. NiSi), combinations thereof, or the like.

As shown in FIG. 1A, the semiconductor device 100 may include first gate stacks 114 disposed adjacent to (e.g. surrounding) the channel layers 110 b of the first protrusions 110, and second gate stacks 214 disposed adjacent to (e.g. surrounding) the channel layers 210 b of the second protrusions 210. For example, the first gate stacks 114 may encircle all sides of the channel layers 110 b of the first protrusions 110, while the second gate stacks 214 may encircle all sides of the channel layers 210 b of the second protrusions 210, e.g. when viewed in a top-down view.

The first gate stacks 114 and the second gate stacks 214 may be disposed over a first spacer layer 116 formed over the first silicide regions 112 and the second silicide regions 212 and around the source layers 110 a and 210 a of the first protrusions 110 and the second protrusions 210. A portion of the first spacer layer 116 may also be disposed over the isolation feature 108, as shown in FIG. 1A. The first spacer layer 116 may be used to provide an insulating layer that prevents the first gate stacks 114 from electrically contacting the underlying first doped region 106. The first spacer layer 116 also prevents the second gate stacks 214 from electrically contacting the underlying second doped region 206.

In some embodiments, the first spacer layer 116 may comprise a dielectric material, such as silicon nitride, for example, formed using any suitable process, such as, CVD, PVD, ALD, and the like. In some embodiments, the deposition of first spacer layer 116 may be a conformal process that is performed after the formation of the first silicide regions 112 and the second silicide regions 212. An etch back process may be subsequently performed to remove excess portions of first spacer layer 116 from the top surfaces of the first protrusions 110 and the second protrusions 210, from sidewalls of the drain layers 110 c and 210 c, and from sidewalls of the channel layers 110 b and 210 b.

Each of the first gate stacks 114 may include a conformal first gate dielectric 114 a and a first gate electrode 114 b formed over first gate dielectric 114 a. In like manner, each of the second gate stack 214 may include a conformal second gate dielectric 214 a and a second gate electrode 214 b formed over second gate dielectric 214 a. The first gate dielectric 214 a and the second gate dielectric 214 a may include silicon oxide, silicon nitride, or multilayers thereof. Additionally or alternatively, the first gate dielectric 114 a and the second gate dielectric 214 a may include a high-k dielectric material. In such embodiments, first gate dielectric 114 a and the second gate dielectric 214 a may include a metal oxide or a silicate of hafnium (Hf), aluminum (Al), zirconium (Zr), lanthanum (La), magnesium (Mg), barium (Ba), titanium (Ti), lead (Pb), combinations thereof, and the like. The first gate dielectric 114 a and the second gate dielectric 214 a may be formed by molecular beam deposition (MBD), ALD, PECVD, and the like.

The first gate electrode 114 b and the second gate electrode 214 b may include a metal-containing material such as titanium nitride (TiN), tantalum nitride (TaN), tantalum carbon (TaC), cobalt (Co), ruthenium (Ru), aluminum (Al), combinations thereof, multi-layers thereof, and the like. In the example shown in FIG. 1A, the first gate electrode 114 b comprises a multi-layer structure that is conformally formed over the first gate dielectric 114 a. However, in other embodiments, the first gate electrode 114 b may comprise a single-layer structure. In the example shown in FIG. 1A, the second gate electrode 214 b comprises a single-layer structure that is conformally formed over the second gate dielectric 214 a. However, in other embodiments, the second gate electrode 214 b may comprise a multi-layer structure.

The first gate electrode 114 b and second gate electrode 214 b in FIG. 1A are conformally formed over the first gate dielectric 114 a and the second gate dielectric 214 a, respectively. However, in other embodiments, the first gate electrode 114 b and the second gate electrode 214 b may not be a conformal structure and, instead, may be formed using a suitable deposition process such as MBD, ALD, PECVD, and the like. In such embodiments, an etch back process may be performed to remove excess portions of the first gate electrode 114 b from top surfaces of the first protrusions 110 and from sidewalls of the drain layers 110 c of the first protrusions 110. This etch back process may also remove excess portions of the second gate electrode 214 b from top surfaces of the second protrusions 210 and from sidewalls of the drain layers 210 c of the second protrusions 210.

The semiconductor device 100 may further include a second spacer layer 118 a, 118 b disposed over the first gate stack 114 and the second gate stack 214. The second spacer layer 118 a, 118 b may comprise an oxide layer 118 a (e.g. silicon oxide or silicon dioxide) and/or a nitride layer 118 b (e.g. silicon nitride). The second spacer layer 118 a, 118 b may be formed using any suitable process, such as, CVD, PVD, ALD, and the like. An etch back process may be performed to remove excess portions of the second spacer layer 118 from the top surfaces of the first protrusions 110 and the second protrusions 210, and from at least a portion of the sidewalls of the drain layers 110 c and 210 c of the first protrusions 110 and the second protrusions 210, as shown in FIG. 1A. Accordingly, the top surfaces of the drain layers 110 c and 210 c and at least a portion of the sidewalls of the drain layers 110 c and 210 c may be exposed and subjected to the process flow steps that follow.

In the process steps that follow, the drain layers 110 c of the first protrusions 110 and the drain layers 210 c of the second protrusions 210 may be enlarged using, for example, an epitaxial growth process. The description that follows shows an example of epitaxially growing the drain layers 110 c of the first protrusions 110 prior to epitaxially growing the drain layers 210 c of the second protrusions 210. However, in another embodiment, the drain layers 110 c of the first protrusions 110 may be epitaxially grown after epitaxially growing the drain layers 210 c of the second protrusions 210. In yet another embodiment, the drain layers 110 c of the first protrusions 110 may be epitaxially grown, while the drain layers 210 c of the second protrusions 210 are kept without enlargement. In still another embodiment, the drain layers 210 c of the second protrusions 210 may be epitaxially grown, while the drain layers 110 c of the first protrusions 110 are kept without enlargement. The relevant process flow steps presented in the description that follows may be applied to these other embodiments.

As shown in FIG. 1B, a first hard mask 120 may be formed (e.g. conformally formed) over the second spacer layer 118, the first protrusions 110, and the second protrusions 210. The first hard mask 120 may completely cover exposed surfaces of the second spacer layer 118 a, 118 b, the first protrusions 110, and the second protrusions 210. Consequently, the first hard mask 120 may cover the exposed sidewalls of the drain layers 110 c of the first protrusions 110 and the exposed sidewalls of the drain layers 210 c of the second protrusions 210.

The first hard mask 120 may include an oxide layer 120 a (e.g. comprising silicon oxide or silicon dioxide) and a nitride layer 120 b (e.g. comprising silicon nitride) formed over the oxide layer 120 a. The oxide layer 120 a and the nitride layer 120 b of the first hard mask 120 may be formed using a suitable process such as chemical vapor deposition, plasma enhanced chemical vapor deposition, atomic layer deposition, or the like. However, other suitable methods of forming the oxide layer 120 a and the nitride layer 120 b of the first hard mask 120 may be utilized. The first hard mask 120 may be formed to a thickness of between about 2 nm and about 60 nm, such as about 40 nm.

Once the first hard mask 120 has been formed, a portion of the first hard mask 120 may be removed in order to expose the drain layers 110 c of the first protrusions 110, while keeping the drain layers 210 c of the second protrusions 210 covered. In other words, the first hard mask 120 may be patterned to expose sidewalls of the drain layers 110 c of the first protrusions 110 and portions of the second spacer layer 118 disposed over the first gate stacks 114, while a remaining portion of the first hard mask 120 continues to cover sidewalls of the drain layers 210 c of the second protrusions 210 and portions of the second spacer layer 118 disposed over the second gate stacks 214. This step is illustrated in FIG. 1C. In an embodiment, a masking and etching process (e.g. dry and/or wet etch process) may be used to expose sidewalls of the drain layers 110 c of the first protrusions 110 and portions of the second spacer layer 118 disposed over the first gate stacks 114. However, it should be understood that other suitable methods of removing a portion of the first hard mask 120 may be utilized in other embodiments.

Referring to FIG. 1D, a third epitaxial growth process 122 may be performed to enlarge the drain layers 110 c of the first protrusions 110, thereby forming first enlarged drain regions 124 over exposed surfaces of the drain layers 110 c of the first protrusions 110. The third epitaxial growth process 122 may be a low-temperature epitaxial growth process, e.g. performed at a temperature in a range from about 400° C. to about 650° C., such as about 465° C. The third epitaxial growth process 122 may be molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), selective epitaxial growth (SEG), or combinations thereof. In an embodiment, the third epitaxial growth process 122 may be performed for a time duration in a range from about 2 minutes to about 90 minutes (e.g. about 15 minutes). This may result in the first enlarged drain regions 124 having a thickness T1 in a range from about 1 nm to about 50 nm (e.g. about 10 nm), although other thicknesses and time durations may be possible. The third epitaxial growth process 122 may include the use of one or more process gases and one or more carrier gases. In an embodiment, the one or more process gases may include silicon chloride hydride (SiCl₂H₂), silane (SiH₄), phosphine (PH₃), combinations thereof, or the like. The one or more carrier gases may include nitrogen (N₂) and/or hydrogen (H₂). Under the process conditions described above, the third epitaxial growth process 122 may have a growth rate in a range from about 0.5 nm per minute to about 3 nm per minute (e.g. about 1 nm per minute).

The first enlarged drain regions 124 may comprise doped semiconductor material having the same conductivity as the first doped region 106 and the drain layers 110 c (e.g. the first conductivity). The dopant concentration of the first enlarged drain regions 124 may be substantially equal to the dopant concentration of the first doped region 106. In an embodiment, dopants are introduced into the semiconductor material of the first enlarged drain regions 124 as the semiconductor material of the first enlarged drain regions 124 is grown. The description given above in respect of introducing dopants into the semiconductor material of the first doped region 106, the source layers 110 a, the channel layers 110 b, and the drain layers 110 c as the semiconductor material of each of these layers is grown may analogously apply to introducing dopants into the semiconductor material of the first enlarged drain regions 124 as the semiconductor material of the first enlarged drain regions 124 is grown.

Alternatively, in another embodiment, dopants may be introduced into the semiconductor material of the first enlarged drain regions 124 after the semiconductor material of the first enlarged drain regions 124 is grown. The description given above in respect of introducing dopants into the semiconductor material of the first doped region 106, the source layers 110 a, the channel layers 110 b, and the drain layers 110 c after the semiconductor material of each of these layers is grown may analogously apply to introducing dopants into the semiconductor material of the first enlarged drain regions 124 after the semiconductor material of the first enlarged drain regions 124 is grown.

As described above, the third epitaxial growth process 122 may form semiconductor material on exposed surfaces of the drain layers 110 c. These exposed surfaces of the drain layers 110 c include the exposed sidewalls and top surfaces of the drain layers 110 c. The growth of the semiconductor material may proceed in a lateral direction (e.g. laterally away from the sidewalls of the drain layers 110 c), in a vertical direction (e.g. in a direction away from the semiconductor substrate 104), or a combination thereof (e.g. in an oblique direction).

FIG. 2A shows a top-down view of the drain layers 110 c and the first enlarged drain regions 124 in an embodiment where the first protrusions 110 are shaped as nanowires. FIG. 2B shows the orientation of the lattice planes (expressed as Miller indices) for various surfaces of one of the drain layers 110 c, when the drain layer 110 c is viewed in a top-down view.

The cross-sectional view of the first enlarged drain regions 124 and the drain layers 110 c shown in FIG. 1D may be a taken along the line A-A′ in FIG. 2A. A magnified cross-sectional view of the first enlarged drain regions 124 and the drain layers 110 c of the first protrusions 110 taken along the line A-A′ in FIG. 2A is illustrated in FIG. 2C. A magnified cross-sectional view of the first enlarged drain regions 124 and the drain layers 110 c of the first protrusions 110 taken along the line B-B′ in FIG. 2A is illustrated in FIG. 2D.

As shown in FIGS. 2B, 2C, and 2D, various surfaces of the drain layers 110 c may have different lattice plane orientations. The growth of semiconductor material on a surface of the drain layers 110 c may depend on the lattice plane orientation of the surface. For example, the growth rate of semiconductor material on a surface having a lattice plane orientation (100) may be greater than the growth rate of semiconductor material on a surface having a lattice plane orientation (110). Furthermore, the growth rate of semiconductor material on a surface having a lattice plane orientation (110) may be greater than the growth rate of semiconductor material on a surface having a lattice plane orientation (111). For example, for the third epitaxial growth process 122, the growth rate of semiconductor material on a surface having a lattice plane orientation (110) may be in an upper range of the aforementioned range of about 0.5 nm per minute to about 3 nm per minute (e.g. about 1.5 nm per minute to about 3 nm per minute), while the growth rate of semiconductor material on a surface having a lattice plane orientation (111) may be in a lower range of the aforementioned range of about 0.5 nm per minute to about 3 nm per minute (e.g. about 0.5 nm per minute to about 1.5 nm per minute). Consequently, the third epitaxial growth process 122 may cause semiconductor material to grow on exposed surfaces of the drain layers 110 c such that the first enlarged drain regions 124 comprise various facets (or faces) F1 to F6 (shown in FIG. 2D) having various orientations and subtending various angles with respect to a reference line R (e.g. a horizontal reference line). As an example, facets F4, F5, and F6 of the first enlarged drain regions 124 proximal bottom regions 110 cb of the drain layers 110 c may extend a distance b from sidewalls of the drain layers 110 c. In some embodiments, the distance b may be in a range from about 0 nm to about 50 nm, such as in a range from about 10 nm to about 40 nm, e.g. about 25 nm. Furthermore, an angle c subtended between these facets F4, F5, and F6 of the first enlarged drain regions 124 and the reference line R (e.g. horizontal reference line) may be in a range from about 0 degrees to about 90 degrees, e.g. in a range from about 30 degrees to about 60 degrees, e.g. in a range from about 35 degrees to about 55 degrees.

As described above, the third epitaxial growth process 122 may be performed for a time duration in a range from about 10 minutes to about 90 minutes (e.g. about 15 minutes). Depending on this time duration, the first enlarged drain region 124 formed over exposed surfaces of a first drain layer 110 c 1 may or may not physically contact the first enlarged drain region 124 formed over exposed surfaces of a second drain layer 110 c 2 laterally adjacent to the first drain layer 110 c 1. For example, if the third epitaxial growth process 122 is applied for a longer duration of time (e.g. greater than about 15 minutes), the first enlarged drain region 124 formed over exposed surfaces of the first drain layer 110 c 1 may physically contact the first enlarged drain region 124 formed over exposed surfaces of a second drain layer 110 c 2 laterally adjacent to the first drain layer 110 c 1 (e.g. as shown in FIGS. 2C and 2D). However, in an embodiment where the third epitaxial growth process 122 is applied for a shorter duration of time (e.g. less than about 15 minutes), the first enlarged drain region 124 formed over exposed surfaces of the first drain layer 110 c 1 may not physically contact the first enlarged drain region 124 formed over exposed surfaces of a second drain layer 110 c 2 laterally adjacent to the first drain layer 110 c 1. In the embodiment where the first enlarged drain regions 124 of adjacent drain layers 110 c physically contact each other, a thickness T2 (shown in FIG. 2D) of the region at which the first enlarged drain regions 124 physically contact each other may be in a range from about 1 nm to about 50 nm, e.g. in a range from about 10 nm to about 40 nm, e.g. about 25 nm.

Referring to FIG. 1E, the process flow continues with masking the first enlarged drain regions 124 (e.g. with a second hard mask 126) and removing the portion of the first hard mask 120 disposed over the drain layers 210 c of the second protrusions 210. This may be accomplished by depositing (e.g. by spin-on coating, chemical vapor deposition, plasma enhanced chemical vapor deposition) the second hard mask 126 over the first enlarged drain regions 124 and the portion of the first hard mask 120 disposed over the drain layers 210 c of the second protrusions 210. The second hard mask 126 may comprise similar materials as the first hard mask 120. Subsequently, a planarizing process (e.g. a chemical mechanical polishing process) may be performed to planarize the second hard mask 126 and expose the portion of the first hard mask 120 disposed over the drain layers 210 c of the second protrusions 210. Following this, an etching process (e.g. a wet and/or dry etch process) may be performed to remove the portion of the first hard mask 120 disposed over the drain layers 210 c of the second protrusions 210, thereby exposing the drain layers 210 c of the second protrusions 210, as shown in FIG. 1E. The planarizing process and the etching process performed on the first hard mask 120 are not shown in the process flow for the sake of brevity.

Referring to FIG. 1F, a fourth epitaxial growth process 222 may be performed to enlarge the drain layers 210 c of the second protrusions 210, thereby forming second enlarged drain regions 224 over exposed surfaces of the drain layers 210 c. The fourth epitaxial growth process 222 may be a low-temperature epitaxial growth process performed at a temperature in a range from about 400° C. to about 650° C., such as about 465° C. The fourth epitaxial growth process 222 may be molecular beam epitaxy (MBE), liquid phase epitaxy (LPE), vapor phase epitaxy (VPE), selective epitaxial growth (SEG), or combinations thereof. In an embodiment, the fourth epitaxial growth process 222 may be performed for a time duration in a range from about 10 minutes to about 90 minutes (e.g. about 15 minutes). This may result in the second enlarged drain regions 224 having a thickness T3 in a range from about 1 nm to about 50 nm (e.g. about 10 nm), although other thicknesses and time durations may be possible. The fourth epitaxial growth process 222 may include the use of one or more process gases and one or more carrier gases. In an embodiment, the one or more process gases of the fourth epitaxial growth process 222 may include silicon chloride hydride (SiCl₂H₂), silane (SiH₄), germane (GeH₄), diborane (B₂H₆), combinations thereof, or the like. The one or more carrier gases may include nitrogen (N₂) and/or hydrogen (H₂). Under the process conditions described above, the fourth epitaxial growth process 222 may have a growth rate in a range from about 0.5 nm per minute to about 3 nm per minute (e.g. about 1 nm per minute).

The second enlarged drain regions 224 may comprise doped semiconductor material having the same conductivity as the second doped region 206 (e.g. the second conductivity). The dopant concentration of the second enlarged drain regions 224 may be substantially equal to the dopant concentration of the second doped region 206. In an embodiment, dopants are introduced into the semiconductor material of the second enlarged drain regions 224 as the semiconductor material of the second enlarged drain regions 224 is grown. The description given above in respect of introducing dopants into the semiconductor material of the first doped region 106, the source layers 110 a, the channel layers 110 b, and the drain layers 110 c as the semiconductor material of each of these layers is grown may analogously apply to introducing dopants into the semiconductor material of the second enlarged drain regions 224 as the semiconductor material of the second enlarged drain regions 224 is grown.

Alternatively, in another embodiment, dopants may be introduced into the semiconductor material of the second enlarged drain regions 224 after the semiconductor material of the second enlarged drain regions 224 is grown. The description given above in respect of introducing dopants into the semiconductor material of the first doped region 106, the source layers 110 a, the channel layers 110 b, and the drain layers 110 c after the semiconductor material of each of these layers is grown may analogously apply to introducing dopants into the semiconductor material of the second enlarged drain regions 224 after the semiconductor material of the second enlarged drain regions 224 is grown.

As described above, the fourth epitaxial growth process 222 may form semiconductor material on exposed surfaces of the drain layers 210 c. These exposed surfaces of the drain layers 210 c include the exposed sidewalls and top surfaces of the drain layers 210 c. The growth of the semiconductor material may proceed in a lateral direction (e.g. laterally away from the sidewalls of the drain layers 210 c), in a vertical direction (e.g. in a direction away from the semiconductor substrate 104), or a combination thereof (e.g. in an oblique direction).

As described above in respect of the drain layers 110 c of the first protrusions 110, various surfaces of the drain layers 110 c may have different lattice plane orientations that can affect the growth of semiconductor material on these surfaces. In a similar manner, various surfaces of the drain layers 210 c of the second protrusions 210 may have different lattice plane orientations (similar to the orientations shown in FIG. 2B). The growth of semiconductor material on a surface of the drain layers 210 c may depend on the lattice plane orientation of the surface. For example, the growth rate of semiconductor material on a surface having a lattice plane orientation (100) may be greater than the growth rate of semiconductor material on a surface having a lattice plane orientation (110). Furthermore, the growth rate of semiconductor material on a surface having a lattice plane orientation (110) may be greater than the growth rate of semiconductor material on a surface having a lattice plane orientation (111). For example, for the fourth epitaxial growth process 222, the growth rate of semiconductor material on a surface having a lattice plane orientation (110) may be in an upper range of the aforementioned range of about 0.5 nm per minute to about 3 nm per minute (e.g. about 1.5 nm per minute to about 3 nm per minute), while the growth rate of semiconductor material on a surface having a lattice plane orientation (111) may be in a lower range of the aforementioned range of about 0.5 nm per minute to about 3 nm per minute (e.g. about 0.5 nm per minute to about 1.5 nm per minute). Consequently, the fourth epitaxial growth process 222 may cause semiconductor material to grow on exposed surfaces of the drain layers 210 c such that the second enlarged drain regions 224 comprise various facets (or faces) having various orientations and subtending various angles with respect to a reference (much like the facets F1 to F6 of the first enlarged drain regions 124, shown in FIG. 2D). Similarly, facets of the second enlarged drain regions 224 proximal bottom regions of the drain layers 210 c may extend by a distance from sidewalls of the drain layers 210 c. In some embodiments, this distance may be in a similar range as the distance b described above in respect of the first enlarged drain regions 124. However, as described above, the second protrusions 210 may be a part or portion of a PMOS VGAA device. In practice, P-type epitaxial crystallization may be easier to perform that N-type epitaxial crystallization. This can result in the distance between sidewalls of the drain layers 210 c and facets of the second enlarged drain regions 224 proximal bottom regions of the drain layers 210 c being smaller than the above-described distance b between sidewalls of the drain layers 110 c and facets of the first enlarged drain regions 124 proximal bottom regions of the drain layers 110 c. However, an angle subtended between these facets of the second enlarged drain regions 224 and a reference line R may be in a similar range as and may be substantially equal to the angle c described above in relation to the first enlarged drain regions 124.

As described above, the fourth epitaxial growth process 222 may be performed for a time duration in a range from about 10 minutes to about 90 minutes (e.g. about 15 minutes). Depending on this time duration, the second enlarged drain region 224 formed over exposed surfaces of a first drain layer 210 c may or may not physically contact the second enlarged drain region 224 formed over exposed surfaces of a second drain layer 210 c laterally adjacent to the first drain layer 210 c. In an embodiment where physical contact is made between adjacent second enlarged drain regions 224, a thickness of the region at which these second enlarged drain regions 224 physically contact may be substantially equal to the thickness T2 (shown in FIG. 2D) described above in respect of the first enlarged drain regions 124.

Referring to FIG. 1G, the second hard mask 126 disposed over the first enlarged drain regions 124 may be removed (e.g. by an etching process) to expose the first enlarged drain regions 124 and the second enlarged drain regions 224.

Referring to FIG. 1H, the process flow may be continued (e.g. in multiple process flow steps) to form a first drain contact 128 over the first enlarged drain regions 124, and a second drain contact 228 over the second enlarged drain regions 224. The first drain contact 128 may include a first drain silicide 128 c disposed over the first enlarged drain regions 124, and first conductive layers 128 b and 128 a disposed over the first drain silicide 128 c. The first drain silicide 128 c may include similar materials as the first silicide regions 112. The first conductive layers 128 b and 128 a may comprise a conductive material such as copper, tungsten, or the like. The second drain contact 228 may include a second drain silicide 228 c disposed over the second enlarged drain regions 224, and second conductive layers 228 b and 228 a disposed over the second drain silicide 228 c. The second drain silicide 228 c may include similar materials as the second silicide regions 212. The second conductive layers 228 b and 228 a may comprise a conductive material such as copper, tungsten, or the like.

Following the manufacture of the structure shown in FIG. 1H, dielectric material (e.g. comprising an oxide and/or a nitride) may be deposited over and may fully cover the first drain contact 128 and the second drain contact 228, e.g. on all sides of the first drain contact 128 and the second drain contact 228. The dielectric material fully covering the first drain contact 128 and the second drain contact 228 may, as an example, form an interlayer dielectric (ILD) layer of the semiconductor device 100.

An effect provided by the process flow illustrated in FIGS. 1A and 1H is a larger contact area between the first drain contact 128 and the drain regions of the first VGAA device 102 compared to an NMOS VGAA device where the first enlarged drain regions 124 are absent (hereinafter referred to as a conventional NMOS VGAA device only for the sake of brevity and convenience). Similarly, the process flow illustrated in FIGS. 1A and 1H leads to a larger contact area between the second drain contact 228 and the drain regions of the second VGAA device 202 compared to a PMOS VGAA device where the second enlarged drain regions 224 are absent (hereinafter referred to as a conventional PMOS VGAA device only for the sake of brevity and convenience). For example, in an embodiment where the first protrusions 110 and the second protrusions 210 are arranged as 2×3 matrices, a contact area between the first drain contact 128 and the first enlarged drain regions 124 may be in a range from about 3000 nm² to about 4000 nm² (e.g. about 3500 nm²). A contact area between the second drain contact 228 and the second enlarged drain regions 224 may be in a similar range. In comparison, a contact area between the drain contact and drain regions of the conventional NMOS VGAA device and the conventional PMOS VGAA device may be in a range from about 1000 nm² to about 2000 nm² (e.g. about 1600 nm²). This increase in contact area between the drain contact and drain regions of the first VGAA device 102 and the second VGAA device 202 in turn leads to lower contact resistances in the first VGAA device 102 and the second VGAA device 202 as well as a larger drain pad landing for the first VGAA device 102 and the second VGAA device 202. The larger drain pad landing for the first VGAA device 102 and the second VGAA device 202 leads to better control of drain pad enclosure windows for the first VGAA device 102 and the second VGAA device 202. It is noted that the contact resistances in the first VGAA device 102 and the second VGAA device 202 may be reduced further by judiciously selecting the semiconductor materials of the first enlarged drain region 124 and the second enlarged drain region 224. For example, in an embodiment where the semiconductor materials of the first enlarged drain region 124 and the second enlarged drain region 224 comprise both silicon and germanium, a higher germanium concentration relative to silicon can further reduce the contact resistances in the first VGAA device 102 and the second VGAA device 202. In another example, the semiconductor materials of the first enlarged drain region 124 and the second enlarged drain region 224 may be devoid of silicon and this can also result in reduced contact resistances in the first VGAA device 102 and the second VGAA device 202. As an illustration, the first enlarged drain region 124 and the second enlarged drain region 224 may comprise pure germanium, a group III-V semiconductor material, or a combination thereof (e.g. a multilayer structure comprising a layer of pure germanium and another layer of a group III-V semiconductor material). In this example, the contact resistances in the first VGAA device 102 and the second VGAA device 202 is also reduced.

FIGS. 3A and 3B show plan views (e.g. top-down views) of the first protrusions 110 or the second protrusions 210. Also shown in FIGS. 3A and 3B are the first enlarged drain regions 124 or the second enlarged drain regions 224 formed over exposed sidewalls of the drain layers 110 c or 210 c, respectively. As shown in FIG. 3A, the first protrusions 110 or the second protrusions 210 may be shaped as nanowires (e.g. having a substantially circular shape) having a diameter D in a range from about 5 nm to about 20 nm, e.g. about 10 nm. In the embodiment of FIG. 3B, however, the first protrusions 110 or the second protrusions 210 may be shaped as bars or fins that have a first lateral extent L1 in a first direction and a second lateral extent L2 in a second direction substantially perpendicular to the first direction. As shown in FIG. 3B, the first lateral extent L1 is different from (e.g. smaller than) the second lateral extent L2. In an embodiment the first lateral extent L1 may be in a range from about 5 nm to about 20 nm (e.g. about 10 nm), while the second lateral extent L2 may be in a range from about 5 nm to about 2000 nm (e.g. about 60 nm). In some embodiments, the second lateral extent L2 may be larger than about 2000 nm.

According to an embodiment presented herein, a method of manufacturing a vertical gate all around device comprises: exposing a top surface and sidewalls of a first portion of a protrusion extending from a doped region, wherein a second portion of the protrusion is surrounded by a gate stack; and enlarging the first portion of the protrusion using an epitaxial growth process.

According to an embodiment presented herein, a method of manufacturing a vertical gate all around device comprises: forming a first doped region over a substrate, the first doped region having a first conductivity and a first protrusion extending away from the substrate; forming a second doped region laterally adjacent to the first doped region, the second doped region having a second conductivity different from the first conductivity and a second protrusion extending away from the substrate; exposing surfaces of a drain layer of the first protrusion, wherein a channel layer of the first protrusion is surrounded by a first gate stack; exposing surfaces of a drain layer of the second protrusion, wherein a channel layer of the second protrusion is surrounded by a second gate stack; and epitaxially growing semiconductor material over the exposed surfaces of the drain layers of the first protrusion and the second protrusion.

According to an embodiment presented herein, a vertical gate all around device comprises: a semiconductor substrate; a doped region over the semiconductor substrate; a protrusion extending from the doped region away from the semiconductor substrate, the protrusion comprising a source layer proximal the doped region, a channel layer disposed over the source layer, and a drain layer disposed over the channel layer; a gate stack encircling the channel layer of the protrusion; and an enlarged drain region disposed over a top surface and extending from sidewalls of the drain layer of the protrusion.

The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure. 

What is claimed is:
 1. A method comprising: forming a doped region in a semiconductor substrate; forming a protrusion extending from the doped region; depositing a first spacer layer over the doped region, the first spacer layer surrounding a first portion of the protrusion; forming a gate stack over the first spacer layer, the gate stack surrounding a second portion of the protrusion; and epitaxially growing an epitaxial region over a third portion of the protrusion, the epitaxial region contacting a top surface and side surfaces of the protrusion, the epitaxial region having an octagonal shape in a first cross-sectional view.
 2. The method of claim 1, wherein the epitaxial region is epitaxially grown surrounding a first source/drain region of the protrusion, wherein the first source/drain region comprises the third portion of the protrusion.
 3. The method of claim 2, further comprising doping the doped region, the first source/drain region, and a second source/drain region of the protrusion to a dopant concentration from 1×10¹⁹ cm⁻³ to 1×10²² cm⁻³ and doping a channel region of the protrusion to a dopant concentration less than 1×10¹⁸ cm⁻³.
 4. The method of claim 2, further comprising doping the doped region and a second source/drain region of the protrusion to a dopant concentration from 1×10¹⁹ cm ⁻³ to 1×10²² cm ⁻³ and doping a channel region of the protrusion and the first source/drain region to a dopant concentration less than 1×10¹⁸ cm⁻³.
 5. The method of claim 1, wherein forming the gate stack comprises: depositing a gate dielectric layer over the first spacer layer and the protrusion; depositing a gate electrode over the gate dielectric layer; and etching back the gate electrode and the gate dielectric layer to expose the top surface and the side surfaces of the protrusion.
 6. The method of claim 5, further comprising: depositing a second spacer layer over the gate stack; and etching back the second spacer layer to expose the third portion of the protrusion.
 7. The method of claim 1, wherein the epitaxial region has a rectangular shape in a second cross-sectional view orthogonal to the first cross-sectional view.
 8. A method comprising: forming a first doped region over a semiconductor substrate; forming a first protrusion extending from the first doped region; forming a first gate stack surrounding the first protrusion; etching the first gate stack to expose a first portion of the first protrusion; depositing a first spacer surrounding the first portion of the first protrusion; etching the first spacer to expose a second portion of the first protrusion; and epitaxially growing a first semiconductor material surrounding the second portion of the first protrusion, the first semiconductor material having a hexagonal shape in a top-down view.
 9. The method of claim 8, wherein forming the first doped region and forming the first protrusion comprises epitaxially growing a semiconductor material over the semiconductor substrate and patterning the semiconductor material.
 10. The method of claim 9, wherein dopants are introduced into the semiconductor material as the semiconductor material is epitaxially grown.
 11. The method of claim 9, wherein dopants are implanted into the semiconductor material after the semiconductor material is epitaxially grown.
 12. The method of claim 8, wherein the first protrusion comprises a first source region, a first channel region, and a first drain region, wherein forming the first protrusion comprises using different epitaxial growth processes to form semiconductor materials in each of the first source region, the first channel region, and the first drain region.
 13. The method of claim 8, further comprising after forming the first protrusion, using a silicide process to form a first silicide region in the first doped region adjacent the first protrusion.
 14. The method of claim 8, further comprising, prior to forming the first gate stack: depositing a second spacer over the first protrusion and the first doped region; and etching back the second spacer to expose a third portion of the first protrusion, wherein the first gate stack is formed surrounding the third portion of the first protrusion.
 15. A method comprising: forming a first doped region over a semiconductor substrate; forming a second doped region over the semiconductor substrate, the second doped region being laterally adjacent the first doped region; forming a first protrusion extending from the first doped region; forming a second protrusion extending from the second doped region; depositing a first gate stack over the first protrusion; etching back the first gate stack to expose a first portion of the first protrusion; epitaxially growing a first epitaxial semiconductor material from the first portion of the first protrusion, the first epitaxial semiconductor material having an octagonal shape in a first cross-sectional plane; depositing a second gate stack over the second protrusion; etching back the second gate stack to expose a second portion of the second protrusion; and epitaxially growing a second epitaxial semiconductor material from the second portion of the second protrusion, the second epitaxial semiconductor material having an octagonal shape in the first cross-sectional plane.
 16. The method of claim 15, further comprising: etching a recess between the first doped region and the second doped region; and depositing an insulating material in the recess to form an isolation feature.
 17. The method of claim 16, further comprising forming a first spacer layer over the semiconductor substrate, the first spacer layer extending from the first protrusion, over the isolation feature, to the second protrusion, wherein the first gate stack and the second gate stack are deposited over the first spacer layer.
 18. The method of claim 17, further comprising forming a second spacer layer over the first gate stack and the second gate stack, the second spacer layer physically contacting the first spacer layer, the second spacer layer extending from a sidewall of the first gate stack to a sidewall of the second gate stack.
 19. The method of claim 18, wherein forming the second spacer layer comprises: depositing a nitride layer over the first gate stack, the second gate stack, the first spacer layer, the first protrusion, and the second protrusion; depositing an oxide layer over the nitride layer; and etching the oxide layer and the nitride layer back to expose the first protrusion and the second protrusion before epitaxially growing the first epitaxial semiconductor material and the second epitaxial semiconductor material.
 20. The method of claim 16, further comprising siliciding portions of the first doped region and the second doped region between the first protrusion and the second protrusion to form a silicide region, wherein the isolation feature extends through the silicide region. 